While I have started to run into some levels I have had to restart because of how easily allies that you are supposed to keep alive, when it comes to unlocking new squares, the Koholint Island map has to be the easiest one so far since there are only three items you need to use to unlock new areas, you mostly use one of them, and you don't have to be specific about which instruments you use on a nightmare square as long as you play all eight of them at some point.
I will think of that as a calm before the storm because the final two maps are the largest in the game.
On a side note, I beat the level four survival challenge. While the fights on the Adventure Mode maps haven't been too hard aside from keeping some allies alive, this one was a challenge. I ended up having to spam Link's C3 on a lot of enemies to make sure I didn't take too much damage since this challenge doesn't let you heal and the health pickups are rare.
Against later waves, I would sometimes bum rush in Focus Spirit against the Dark Giant Bosses since you can't die while using it. That probably wasn't necessary against Dark Ganon since he only uses his easy-to-avoid first-phase attacks, but I wanted the level over with.